BACKGROUND ART
Piezoelectric materials have become more and more widely used in a large number of applications. For example, piezoelectric materials have the potential of allowing aircraft designers to minimize the number of required moving parts with high precision as well as increased compactness. Conventional piezoelectric materials, however, generally only work well in applications that require micro-displacement such as adaptive optics, printer jet control, pressure and acoustic transducers. It is therefore desirable to provide a system and method which is readily adaptable to application structures such as aircraft and automobiles which require large displacements (or strokes) under high loads. Such a system would gain widespread acceptance in both the aviation and automotive industries.
Due to the limited strain capability (i.e. elongation per unit voltage input) of piezoelectric materials, a number of piezoelectric segments are typically fixedly coupled or glued together to obtain a useful displacement. In addition to the piezoelectric stack material, a stack supporting component is typically employed to prevent the piezoelectric stack from becoming laterally, vertically, or rotationally unstable. An amplification arm is further included to couple the piezoelectric stack to an external load as well as magnify the displacement.
An infinitely stiff actuator assembly is desired because it directly effects the actuator assembly performance. In fact, surveys of piezoelectric actuators show that more than 50% of actuator assembly compliance is due to stack longitudinal stiffness. It is further known that the cross-sectional area of the piezoelectric stack determines the amount of longitudinal stiffness. It is therefore desirable to provide a piezoelectric actuator assembly that has a very large piezoelectric stack cross-sectional area.
Conventional piezoelectric actuator assembly designs have to accommodate the actuator assembly output displacement in addition to the cross-sectional area of the piezoelectric driver assembly. Therefore, as output displacement requirements increase, there is less space available for the stack cross-sectional area. Correspondingly, if the cross-sectional area of the piezoelectric driver assembly is increased, less output displacement is available. This shortcoming associated with conventional designs is largely due to the fact that typical approaches combine the stack supporting component with the amplification arm such that the two interfere with one another. It is therefore desirable to provide an actuator assembly that has an amplification arm extending in the outboard direction with respect to the attached application structure such that the piezoelectric driver assembly can have an increased cross-sectional area.
A further concern with aircraft, automotive, and other high-force applications is inboard mounting of the actuator assembly. For example, in helicopter rotor blade applications, it is desirable to minimize the effect of external loads due to blade "flapping" motions. Thus, fixing the internal mount in the lateral direction while establishing a nonlinear spring constraint in the axial and vertical directions would enhance aircraft performance and ultimately reduce costs.
Another problem associated with implementing piezoelectric actuators in particular, is the need to preload the piezoelectric stack material. This requirement is due to the fact that piezoelectric materials generally operate best under compression of the material. Therefore, to fully capture the displacement effectiveness of the piezoelectric material, the material should be preloaded to a point of slight compression. It is therefore desirable to provide a piezoelectric actuator assembly that is preloaded to improve displacement amplification and response time.